1. Field of the Invention
The present invention relates to a pattern inspection apparatus, a pattern inspection method, or a program for causing a computer to execute the method. For example, it relates to a pattern inspection technique for inspecting a pattern defect of an object serving as a target workpiece used for semiconductor manufacture, and to an apparatus for inspecting a defect of an extremely small pattern, such as a photomask, a wafer, or a liquid crystal substrate, used when manufacturing a semiconductor element or a liquid crystal display (LCD) and an inspection method thereof.
2. Description of Related Art
In recent years, with an increase in high integration and large capacity of large-scale integrated (LSI) circuits, a circuit line width required for semiconductor elements is becoming narrower and narrower. These semiconductor elements are manufactured by exposing and transferring a pattern onto a wafer to form a circuit by means of a reduced projection exposure apparatus, known as a stepper, while using a master or “original” pattern (also called a mask or a reticle, and hereinafter generically referred to as a mask) with a circuit pattern formed thereon. Therefore, in order to manufacture a mask for transfer printing a fine circuit pattern onto a wafer, an electron beam pattern writing apparatus capable of writing or “drawing” fine circuit patterns needs to be employed. The pattern circuits may be directly written onto a wafer by the pattern writing apparatus. They are written using electron beams or laser beams, for example.
Since a lot of manufacturing cost is needed for the production of LSI, an improvement in yield is a crucial issue. However, as typified by a DRAM (Dynamic Random Access Memory) of 1 giga-bit class, the order of a pattern constituting the LSI has been changing from submicron to nano-meter. Then, one of major factors that decrease the yield is a pattern defect of a mask used in exposing and transferring an ultrafine pattern onto a semiconductor wafer by a photolithography technique. In recent years, with miniaturization of an LSI pattern formed on a semiconductor wafer, dimensions to be detected as a pattern defect have become extremely small. Thus, a pattern inspection apparatus for inspecting defects of a transfer mask used in manufacturing the LSI needs to be highly accurate.
With development of multimedia technologies, the size of a liquid crystal substrate of an LCD (Liquid Crystal Display) is becoming larger, e.g., 500 mm×600 mm or more, and a pattern of a TFT (Thin Film Transistor) or the like formed on the liquid crystal substrate is becoming finer. Therefore, it is increasingly required to inspect an ultra-fine pattern defect in a large range. For this reason, it is urgently required to develop a pattern inspection apparatus which efficiently inspects defects of a pattern of a large-area LCD and a photomask used in manufacturing the large-area LCD in a short time.
Regarding a conventional pattern inspection apparatus, it is known that inspection is performed by comparing an optical image obtained by capturing a pattern formed on a target workpiece or “sample” such as a lithography mask at a predetermined magnification by use of a magnification optical system with design image data or with an optical image of an identical pattern on the target workpiece. For example, the following is known as pattern inspection methods: “die to die inspection” which compares optical image data obtained by capturing images of identical patterns at different positions on the same mask, and “die to database inspection” which, based on writing data (data on a design pattern) generated by converting pattern CAD data used when writing a mask pattern into an appropriate format for input to an inspection apparatus, generates design image data as reference image data, and compares the reference image data with optical image data serving as measurement data obtained by capturing the image of the pattern. When inspecting using the inspection apparatus, the target workpiece is placed on a stage to be scanned by a flux of light while the stage is moving to perform inspection. The target workpiece is irradiated with a flux of light from a light source and an illumination optical system. Light transmitted through the target workpiece or reflected therefrom is focused on a sensor through the optical system. The image captured by the sensor is transmitted to a comparison circuit as measurement data. In the comparison circuit, after position alignment of the images, the measurement data and the reference data are compared based on an appropriate algorithm. If there is no matching between them, it is judged that a pattern defect exists.
The reference image is generated by performing a filter calculation for a multiple value image developed from design data by using a model function in which the mask and optical characteristics are modeled. However, for example, when defects of a micro mask pattern such as an assistant pattern are inspected, it is necessary to generate a highly precise reference image. For this reason, the resolution of the multiple value image developed from design data is increased to N times the resolution of the optical image, for example. N is, for example, an integer of two or more. Then, there has been examined a method of estimating coefficients of a model function by using the multiple value image with high resolution and a sample image for learning. According to this method, a reference image to be used for the inspection is generated by performing a filter calculation for the multiple value image developed from the design data corresponding to an image to be actually inspected, by using the estimated coefficients. Then, highly precise inspection is performed by comparing this reference image with an actual optical image.
The sample image used for estimating the coefficients has only the data of the same pixel unit as that of the actual optical image. Accordingly, it becomes necessary to also change the resolution of the sample image to N times in order to make it in accord with the resolution of the reference image which has been increased to N times. However, the sample image, whose resolution has been changed to N times, has lost the original high frequency components included in a multiple value image developed from the design data. Therefore, if coefficients of a model function are estimated by using a sample image in which the high frequency components have been lost and a multiple value image developed from design data having high frequency components, there is a problem when a reference image of an actual image is generated that the reference image is blurred compared to the actual image. As a result, there is a problem of erroneously detecting “defects” (pseudo defect detection) which are not actually defects.
Regarding generating a reference image, a technique is disclosed that performs a filtering process for rounding corner portions of a pattern (refer to e.g., Japanese Patent Application Laid-open (JP-A) No. 2005-338666). However, this processing relates to a filtering process for sufficiently precisely inspecting pattern corner portions, and does not solve the problem described above.
As mentioned above, there is a problem of generating a reference image which is blurred compared to an actual image because of estimating coefficients of a model function by using a sample image in which high frequency components have been lost and a multiple value image developed from design data having high frequency components. Consequently, there is a problem of erroneously detecting “defects” (pseudo defect detection) which are not actually defects.